This invention relates generally to bipolar transistors and in particular to silicon/silicon-germanium heterojunction bipolar transistors.
There has been a continuing need for transistors that can operate at higher and higher frequencies in such applications as high-speed communications and digital data processing. Bipolar silicon transistors, which find many uses in communications and digital electronic circuits, have not adequately met this need. Such a transistor must have a base region that is as thin as possible to obtain the highest possible unity-gain cutoff frequency and that has as little resistance as possible to achieve the highest possible frequency of oscillation. These requirements are at odds with each other because the resistance of the base goes up as its thickness decreases. A related problem is that inter-element capacitances, especially base-to-emitter capacitances, have limited the maximum frequencies of bipolar transistors
A heterojunction bipolar transistor ("HBT"), for example a transistor having a junction between a silicon ("Si") emitter and a silicon-germanium ("SiGe") base, is largely free of the inverse relationship between base thickness and base resistance. In an HBT transistor the base doping can be increased without degrading the emitter injection efficiency, thus increasing the maximum frequency and reducing the physical size and the inter-element capacitances of the transistor.
Research respecting Si/SiGe HBTs has been conducted by workers at Stanford University in collaboration with workers at Hewlett-Packard Company, the assignee of this application. This research is described in Gibbons, J. F., et al., "Si/Si.sub.1-x Ge.sub.x Heterojunction Bipolar Transistors Fabricated by Limited Reaction Processing," Technical Digest, 1988 International Electron Devices Meeting, San Francisco, Calif. Dec. 11-14, 1988, pp. 566-569, and in King, C. A., et al., "Si/Si.sub.1-x Ge.sub.x Heterojunction Bipolar Transistors Produced by Limited Reaction Processing," IEEE Electron Devices Letters, February, 1989, Vol. 10, pp. 52-54. The limited reaction processing which is described in these papers provides one method of epitaxial deposition of silicon or silicon-germanium. This method involves a chemical vapor deposition technique in a lamp-heated system; the method permits the wafer to be heated and cooled quickly at the beginning and end of each deposition step.
Background work respecting Si/SiGe HBTs is also described in other publications such as Narozny, P., et al., "Si/SiGe Heterojunction Bipolar Transistor With Graded Gap SiGe Base Made by Molecular Beam Epitaxy", IEDM, IEEE, 1988, pp. 562-565. Further efforts at developing improved bipolar transistors are described, for example, in U.S. Pat. No. 4,897,704 issued to Sakurai on Jan. 30, 1990, and in U.S. Pat. No. 5,175,603, issued to Hamasaki on Dec. 29, 1992.
In spite of the advances in the an that are described in the above references, there remains a need for higher-frequency bipolar transistors with lower inter-element capacitances than have been attained.